A primary purpose of a vehicle's suspension system is to provide vertical compliance between the road and the chassis, in order to isolate the chassis occupants from the roughness in the road and to maintain tire contact with the road, thus providing a path for transferring forces from the bottom of the tire to the chassis, such as to change the speed or direction of the vehicle. Examples of some common independent suspension linkages are known generally as strut & link (also called MacPherson strut), double A-arm (also called double wishbone or SLA), semi-trailing arm, and multi-link.
Each wheel assembly is connected to the chassis by one or more links. A link is defined as a substantially rigid member with a joint or joints at each end that allows a particular motion to take place. It is these links that control the motion (or path) of the wheel as it moves up and down over road bumps. These links also have to transmit the forces generated at the tire-road interface to the chassis.
In an active suspension, controlled forces are introduced to the suspension, such as by hydraulic or electric actuators, between the sprung mass of the vehicle body and its occupants, and the unsprung mass of the wheel assemblies. The unsprung mass is the equivalent mass that reproduces the inertial forces produced by the motions of those parts of the vehicle not carried by the suspension system. This primarily includes the wheel assemblies, any mass dampers associated with the wheel assemblies, and some portion of the mass of the suspension links. The sprung mass is the mass of those parts of the vehicle carried by the suspension system, including the body. Active suspension systems preferably are able to introduce forces that are independent of relative wheel motions and velocities.
U.S. Pat. No. 4,981,309 discloses an active suspension system employing electro-magnetic actuators at each wheel assembly of a rolling vehicle. U.S. Pat. No. 6,364,078, and EP publication 0982162, published Mar. 1, 2000, together disclose a mass damper useful with such electromagnetic suspension actuators and that can move independent of the wheel assembly, but only in a substantially vertical direction. In all other directions, the mass of the mass damper is effectively added to the inertia of the unsprung mass. The entire contents of the above U.S. patents are incorporated herein by reference as if set forth in their entirety.
Generally, all kinematically-induced wheel forces are either forces created by the interaction between the tires and the road, or inertia forces generated by the motion of the unsprung mass. The forces occurring between the tires and road are transferred via the suspension system to the body. Horizontal tire patch forces include both a lateral (i.e., side-to-side) component and a longitudinal (i.e., fore-aft) component. On a smooth road, the longitudinal component is predominantly a result of rolling friction against the road surface, and the lateral component a result of steering. On a non-smooth road, motion of the wheels up and down with respect to the body can introduce other lateral and longitudinal loads at the tire patch, as a result of suspension geometry.
The static toe angle of a wheel, measured at a specific height of the wheel relative to the chassis, is the angle between the central longitudinal axis of the vehicle and the line intersecting the center plane of one wheel with the road surface. A wheel is “toed-in” if the forward portion of the wheel is turned toward the vehicle's central longitudinal axis, and “toed-out” if turned away. It is desirable that the static toe angle be very close to zero degrees at speed, to reduce tire wear and rolling resistance. It is also important for handling considerations whether the toe angle, which is normally set when the vehicle is stationary, changes with speed, roll, pitch or wheel jounce and rebound. Roll is the rotation of the vehicle body about a longitudinal axis of the vehicle, such as is induced during sharp cornering, especially with very soft suspension rates. Pitch is rotation of the body about the lateral axis of the vehicle, such as is induced by heavy braking or acceleration. Jounce is the relative displacement of the wheel upward toward the body from the static condition, typically compressing the suspension springs, while rebound is the relative displacement of the wheel downward, away from the body, from the static condition. As the wheel moves up and down relative to the body, the center of the wheel moves relative to the body along a path called the ‘wheel center locus path’ that in most standard suspensions is non-linear and is most significantly determined by suspension geometries.
Active suspensions can be employed to minimize vertical forces transferred to the body from the wheels, such as by actively moving the wheels relative to the body in such a way that as the wheel goes over a bump in the road, for example, the wheel moves up and down but the vehicle body, as viewed from outside the car, does not. Such an effect requires actively introducing a force between the wheel and body through the suspension actuator. As a result, in an active suspension the wheel center will tend to travel along its locus path a greater overall distance (i.e., spend more time in more significant jounce and rebound positions) than in a typical passive suspension that must rely solely on dampers and spring force to keep the suspension from reaching the end of its travel and ‘bottoming out’. As a result, extreme jounce and rebound of the wheel can occur more frequently in active suspension vehicles, as the suspension works to keep the body steady.
Improvements in active suspension configuration are generally needed, particularly in light of the challenges introduced by active control and the resulting jounce/rebound intensity.